Billie Jean King (born November 22, 1943, in Long Beach, California) is a retired tennis player from the United States. During her career, she won 12 Grand Slam singles titles and 25 Grand Slam doubles titles. She is considered by many to be one of the greatest tennis players and female athletes in history. King was an outspoken advocate against sexism in sports and in society in general. The match for which she is best remembered is the Battle of the Sexes in 1973, in which she defeated the former Wimbledon men's champion Bobby Riggs.

King was born Billie Jean Moffit in 1943. She was the daughter of a firefighter father and homemaker mother. Her younger brother Randy Moffit went on to become a pitcher for the San Francisco Giants. She learned to play tennis on the public courts of Long Beach, California and first gained international recognition in 1961 when, aged 17, she won the women's doubles title at Wimbledon (partnering Karen Hantze Susman). In 1965, she married law-student Lawrence King.

In 1966, King won the first of six singles titles at Wimbledon and reached the World No. 1 ranking for the first time. She followed this up by winning the singles titles at both Wimbledon and the US Championships in 1967. She developed a reputation as an aggressive, hard-hitting net-rusher, with excellent speed and a highly-competitive nature.

King was a significant force in the opening of tennis to professionalism. Prior to the advent of the Open era in 1968, she had to get by on US$100 a week as a playground instructor and student at Los Angeles State College in between playing at major tennis tournaments. In 1967, she attacked the United States Lawn Tennis Association in a series of press conferences, denouncing what she called the association's practice of "shamateurism", where top players were paid under the table to guarantee their entry into tournaments. King argued that this was corrupt and kept the game highly elitist. When the Open era began, King campaigned for equal prize money in the men's and women's games. As the financial backing of the women's game improved, King became the first woman athlete to earn over US$100,000 in prize money in 1971. But inequalities continued to exist. In 1972, King won the US Open but received US$15,000 less than the men's champion Ilie Nastase. She stated that if the prize money was not equal by the following year, she would not play. In 1973, the US Open became the first major tournament to offer equal prize money for men and women.

Despite all King's achievements at the world's biggest tennis tournaments, it is a win over a 55 year-old man in 1973 for which she is best remembered. Bobby Riggs had been a top men's player in the 1930s and 40s. He had then gone on to become a well-known tennis hustler who made a living promoting himself playing in challenge matches. In 1973 he took on the role of male chauvinist and, claiming that the women's game was so inferior to the men's game even a 55 year-old like him could beat the current top female players, he challenged an unprepared Margaret Court to a match and beat her 6-2, 6-1. King, who previously had rejected challenges from Riggs, then decided accepted a lucrative financial offer to play him at the Houston Astrodome in Texas on September 20th 1973, in an event dubbed the Battle of the Sexes. The match drew huge publicity. In front of 30,492 spectators and a worldwide television audience estimated at 50 million people in 37 countries, King beat Riggs 6-4, 6-3, 6-3. The match is considered to be a very singificant event in developing greater recognition and respect for women's tennis.

King was instrumental in establishing the women's tennis tour in the 1970s, and worked tirelessly to promote it. She became the first President of the women's players union – the Women's Tennis Association (WTA) – in 1973. In 1974, she founded Womensports magazine, started the Women's Sports Foundation. She also helped to found World Team Tennis.

King's triumph at the French Open in 1972 made her only the fifth woman in tennis history to win the singles titles at all four Grand Slam events. She also won all four of the mixed doubles titles, and in women's doubles only the Australian Open eluded her. She won a record 20 career titles at Wimbledon – 6 singles, 10 women's doubles, and 4 mixed doubles (this record has since been equalled by Martina Navratilova). She is also the only woman to have won the US Open singles title on all four surfaces on which it has been played (grass, clay, indoor, and hard). In 1973, King became the oldest player to win a professional title when she won at Birmingham. She retired from competitive play later that year after reaching the semi-finals in her last appearance at Wimbledon. During her career, King won 67 professional and 37 amateur singles titles and helped the US win the Fed Cup 7 times. Her career prize money totalled US$1,966,487.

King was inducted into the International Tennis Hall of Fame in 1987. In 1990, Life magazine named her one of the '100 Most Important Americans of the 20th Century'.

In 1971, King began an affair with her secretary Marilyn Barnett. When this came to light in a lawsuit ten years later, King acknowledged the affair and thus became the first American athlete to openly admit to having a homosexual relationship. She received an award from GLAAD - an organisation devoted to reducing discrimination against gays, lesbians and bisexuals - in 2001 for "furthering the visibility and inclusion of the community in her work". The award noted her involvement in production and the free distribution of educational films, as well as serving on the boards of several AIDS charities.

King currently resides in New York and Seattle. In the mid-1980s, she divorced Lawrence King.

The Elton John song "Philadelphia Freedom" is a tribute to King.